1. Field of the Invention
The present invention relates to a method and related apparatus for improving Hybrid Automatic Repeat Request (HARQ) operation for a user equipment in a wireless communications system, and more particularly, to a method and related apparatus for deciding whether retransmission data is combined with data of a previous transmission according to a decoding result of the data of the previous transmission, for enhancing transmission efficiency of the HARQ operation.
2. Description of the Prior Art
The third generation (3G) mobile telecommunications system, the Universal Mobile Telecommunications System (UMTS), has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA provides high frequency spectrum utilization, universal coverage, and high quality, high-speed multimedia data transmission. The WCDMA method also meets all kinds of QoS (Quality of Service) requirements simultaneously, providing diverse, flexible, two-way transmission services and better communication quality to reduce transmission interruption rates. Through the 3G mobile telecommunications system, a user can utilize a wireless communications device, such as a mobile phone, to realize real-time video communications, conference calls, real-time games, online music broadcasts, and email sending/receiving. However, these functions rely on fast, instantaneous transmission. Thus, targeting the third generation mobile telecommunication technology, the prior art provides High Speed Package Access (HSPA) technology, which includes High Speed Downlink Package Access (HSDPA) and High Speed Uplink Package Access (HSUPA), to increase bandwidth utility rate and package data processing efficiency to improve uplink/downlink transmission rate.
For HSDPA and HSUPA, the 3rd Generation Partnership Project (3GPP) provides a Continuous Packet Connectivity (CPC) protocol specification, which includes features that, for user equipments (UEs) in CELL_DCH state, aim to significantly increase the number of packet data users for a cell, reduce the uplink noise rise and improve achievable download capacity for VoIP. For an HSDPA UE, the UE can work in a special mode, HS-SCCH less operation, included in CPC, for reducing the loading on high speed shared control channel (HS-SCCH). In addition, HSDPA and HSUPA adopt Hybrid Automatic Repeat Request (HARQ) technology to enhance retransmission rate and reduce transmission delay. HARQ is a technology combining Feed-forward Error Correction (FEC) and ARQ methods, which makes a Universal Terrestrial Radio Access Network (UTRAN) trigger retransmission according to positive/negative acknowledgement signals (ACK/NAK) transmitted from the UE, and the UE stores useful information about the last failed transmission for later use.
In the HS-SCCH less operation, the HS-SCCH is not transmitted with the first HARQ transmission. In this situation, the UE receives a high speed physical downlink shared channel (HS-PDSCH) according to predefined control information received when the HS-SCCH less operation is enabled, for reducing HS-SCCH overhead. Note that, in the HS-SCCH less operation, the same data is limited to 3 transmissions at most; in other words, the HARQ procedure is limited to 2 retransmissions. The UTRAN has to transmit corresponding control information for retransmissions on the HS-SCCH so that the UE receives the HS-PDSCH according to the corresponding control information.
For a detailed HARQ procedure for the HS-SCCH less operation, please refer to the associated 3GPP communications protocol specification. When the UE receives the data of the first transmission, the UE places the data of the first transmission in a soft buffer, replacing any data previously stored in the soft buffer. Next, the UE decodes the data in the soft buffer according to the predefined control information received when the HS-SCCH less operation is enabled. If the data has been decoded successfully, the UE reports a positive acknowledgement (ACK) to the base station, also known as Node-B, and waits for a new transmission. If the data has not been successfully decoded, the UE does not report a negative acknowledgement (NAK) to the Node-B and just waits for the second transmission, also called the first retransmission. During the second and the third transmissions, the control information, such as “second or third transmission” and “pointer to the previous transmission”, are transmitted on the HS-SCCH.
Next, for the second transmission, the UE receives the data of the first transmission in the soft buffer according to the received control information, “second or third transmission” and “pointer to the previous transmission”, and combines the data of the first transmission with the data of the second transmission for generating a first combined data. Then, the UE decodes the first combined data according to the corresponding control information received in the second transmission. If the first combined data has been decoded successfully, the UE reports an ACK to the Node-B and waits for a new transmission. Otherwise, the UE stores the first combined data in a soft buffer, reports a NAK to the Node-B and waits for the third transmission. The process of the third transmission is identical to the process of the second transmission and is not given here.
From the above, it is known that when the Node-B does not receive an ACK corresponding to the first transmission, the Node-B will start the second transmission. Or, when the Node-B receives a NAK corresponding to the second transmission, the Node-B will start the third transmission. Even if the UE decodes the data of the first transmission or the data of the second transmission successfully and reports a corresponding ACK, the Node-B may not receive the ACK or may decode the ACK as a NAK due to poor communication quality in the environment, and thereby starts a retransmission. On the other hand, the UE decodes the retransmission data after combining the retransmission data with the data previously stored in the soft buffer. In this situation, even if the UE decodes the data of the first transmission or the data of the second transmission successfully, the UE still has to combine retransmission data with the data that has been decoded successfully. Therefore, the data combination is unnecessary and reduces transmission efficiency.
In conclusion, when the UE has sent an ACK but the Node-B does not receive the ACK or decodes the ACK as a NAK, the UE still has to combine retransmission data with the data that has been decoded successfully. As a result, transmission efficiency of the HARQ operation is reduced for the UE in the HS-SCCH operation.